The present invention relates generally to a fiber optic phase shifter and intensity modulator and more particulary to fiber optic phase shifters and modulators that utilize a microbend transducer.
The ability to shift the phase of light propagating in a single mode fiber is quite useful in fiber optic sensors and may also be used in fiber-optic communications. A conventional way to shift the phase of light propagating in a single mode fiber is by stretching the fiber. This is done by wrapping and gluing the fiber around a cylinder of piezoelectric material. When a voltage is applied to the material, the cylinder expands thereby stretching the fiber. Long lengths on the order of 10 meters of fiber and large voltages are needed to drive the piezoelectric cylinder.
The ability to modulate the intensity of light propagating in a optic fiber is also useful in fiber optic communication and sensing systems. Such modulation can be performed by a device external to the fiber such as an electrooptic modulator formed in a lithium niobate crystal. However, in some uses it is particularly advantageous to perform the modulation on the light as it propagates through the fiber without the use of an external modulating means. This modulation is generally performed by bending the fiber or wrapping it around a small-diameter cylinder. However, these systems have limited communications applications because they require motion of the fiber over large distances and hence are very slow (less than 1 Hz in bandwidth).
Microbend transducers have been used to modulate the intensity of light propagating in highly multimode optical fibers. The periodic perturbation causes the light to couple between adjacent modes in succession until light is finally coupled to radiation modes. However, because this process requires repeated coupling between guided modes prior to coupling to radiation modes it requires a relatively large displacement of the microbend transducer to effect intensity modulation.